This invention relates to a Stark effect spectrophone for continuous absorption spectra monitoring, and more particularly to a Stark effect spectrophone for continuous absorption spectra monitoring having divergent field electrodes, and methods of using it.
Spectrophones have become very promising for gas analysis in recent years. See "Laser Optoacoustic Spectroscopy--A New Technique for Gas Analysis by L. B. Kreuzer," Analytical Chemistry, Vol. 46, No. 2., February 1974 pp 239A-244A, and "Excited-State Spectroscopy of Molecules Using Opto-acoustic Detection" by C. K. N. Patel, et al., Physical Review Letters, Vol. 38, No. 21, May 1977, pp 1204-1207. Briefly, an optoacoustic detector or spectrophone operates by sensing pressure pulses induced in a gas sample by absorption of chopped laser radiation passing through it. The laser is tuned to different wavelengths to detect the presence of different constituent gases in the sample. Assuming that no energy is absorbed by the sample cell, the energy absorbed by the sample during each light pulse will increase the pressure of the gas in the sample cell in proportion to the amount of the absorbing gas present. This pressure pulse may be detected by a pressure transducer, hence the term optoacoustic detector for the sample cell and pressure transducer. The problem is that in practice the windows through which the laser beam passes through the cell will absorb some energy and introduce acoustical noise in the system.
To solve this problem, Jack S. Margolis and Michael S. Shumate devised a spectrophone as described in an application Ser. No. 938,297 filed Aug. 31, 1978. Briefly, a CW laser beam having a spectral line coincident with an absorption line of a constituent of a gas sample in a closed cell is directed through windows in the cell to interact with the absorbing constituents. The interaction between the laser and an absorbing constituent causes a pressure proportional to the absorption coefficient of a constituent. Many gases possess a dipole moment, and will consequently have absorption frequencies that will exhibit the Stark effect. The pressure is detected by a microphone while an electric field between two parallel plates is modulated to move the spectral absorption line in and out of coincidence with the laser line. This is accomplished by a DC power source biasing the plates while a modulating waveform generator modulates the bias.
This Stark cell modulation technique moves the absorption line of the absorbing constituent inside the cell across the laser spectral line of interest, producing a pressure increase due to heating, and alternately to a position away from the laser line. The difference in pressure between the ON and OFF condition indicates the presence of the constituent, and the amplitude of the peak indicates the quantity (parts per million) of the constituent. The presence of all constituents of interest may thus be detected and measured by varying the DC bias voltage because the electric field dependence of the microphone signal will be due to absorption of any resonating laser lines coincident with the gas constituents in the spectrophone cell at a specific field intensity. Alternatively, a multiline laser may be used to excite the spectrophone cell, in which case the electric field dependence of the composite response produces a composite absorption which is unique and therefore determines the combination of excited constituents in the spectrophone cell, i.e., produces a signature of the particular gas in the spectrophone cell.
Stark cell spectrophones in the prior art have had parallel plates to give a uniform electric field of a single value between the plates. When the cell is filled with a gas and it is desired to plot the absorption over a range of electric fields, it is necessary to keep changing the value of the voltage on the plates, and take discrete measurements of the microphone output for each value of the voltage on the plates. To plot absorption vs. electric field, the voltage on the plates can be swept simultaneously with readout of the microphone output. This procedure is satisfactory for a static (nonflowing and nonchanging) gas volume, but it is not fast enough for realtime detection of the constituents of a gas flowing through the cell. What is required is a spectrophone which allows realtime detection of the constituents of a gas, instead of sequentially setting various field intensities as is done in the prior art. Thus, in some applications, it would be advantageous to monitor a continuous flow of an unknown gas to determine its constituents, or to otherwise determine the presence of all constituents of interest simultaneously, instead of one at a time by varying the electric field. It is therefore an object of this invention to provide a gas monitoring system for realtime monitoring of a continuous gas flow to eliminate operational problems inherent in varying the field intensity across the plates of the spectrophone, and one which will provide an instantaneous plot of absorption vs. electrostatic field. For example, in monitoring a continuous flow of two or more gases, each of which absorb at the laser wavelength but at slightly different values of electric field strength, the relative concentrations of each gas could be continuously displayed in realtime. This requires an optoacoustic detector having the capability of determining energy absorptions at several field strength values simultaneously.